Next time it happens, turn off the car immediately, then remove the belts, rotate all pulleys by hand and see if they move freely (other than crank of course). If all check out and don't wobble or feel strange, then put the belts back on and check to make sure it's tight.

Also, if you haven't checked the condition of the belts, make sure they aren't glazed and still feel rubbery where the belt meets the pulleys. Check for leaks that could be putting oil, coolant, etc. on the belt.